In the fields of applications of exterior coatings to architectural structures, wheeled vehicles, ships, aircraft, etc., there are utilized high-quality polyester or acryl type exterior coatings, since the coatings used for the purpose intended are required to have excellent weathering resistance and performance properties. However, outdoor durable years of the coatings are short and even the above-mentioned high-quality coatings will lose their fine appearance and substrate protective properties in several years.
Because of their being excellent in thermal and chemical stability, weathering resistance, water resistance, chemical resistance, solvent resistance, non-adhesive properties, low friction characteristics and water repellency, fluorine containing polymers are suitable for use as surface treatment agents for various kinds of substrates or base materials. However, on account of their excellent properties as mentioned above, conventional known fluorine-containing polymers are difficult to use as coating materials because they are not readily soluble in organic solvents and it is hard to form films thereof. For instance, most of the coating materials of fluorine-containing polymers known at present are powder coatings except for PVdF (polyvinylidene fluoride) which is used as an organic solvent dispersible type coating by utilizing its properties of being soluble in specific solvents at elevated temperatures. Moreover, since coating materials of fluorine-containing polymers mentioned above require high temperature baking at the time when their films are formed, the field in which they are used is limited to such places where heating equipment can be arranged. Furthermore, such heating equipment and baking step as required for the utilization of these coating materials are undesirable from the standpoint of the safety of workers engaged in the coating operation or the work environment. Consequently, various attempts have been made in recent years to develop fluorine-containing polymers which are soluble in solvents or which do not require a high temperature baking step.
For example, Japanese Patent Laid-Open Publn. No. 34107/1982 discloses quaternary copolymers comprising fluoroolefin, cyclohexyl vinyl ether, alkyl vinyl ether and hydroxyalkyl vinyl ether, and said copolymers being alleged to be soluble in organic solvents and capable of curing at ordinary temperature. However, for curing these copolymers at ordinary temperature, it is necessary to use melamine type hardeners or urea resin type hardeners, and the use of these hardeners results in lowering in weathering resistance. Furthermore, the glass transition temperature of these copolymers are relatively high such as above ordinary temperature, i.e. 25.degree. C. Moreover, Japanese Patent Publn. No. 39472/1971 suggests, though no concrete example is shown, that polymers obtained by mechanically treating PTFE (polytetrafluoroethylene) with organosilicon compounds having olefinically unsaturated bonds and hydrolyzable groups in the presence of such radical initiators as organic hydroperoxide are crosslinked and cured with water at ordinary temperature. The fluorine-containing polymers according to the above process, however, are polymers in which the organosilicon compounds have grafted on PTFE and are substantially insoluble in organic solvents.
Under the circumstances, the present inventors proposed, in Japanese Patent Application No. 263017/1984, solvent-soluble fluorine-containing copolymers and coating compositions comprising such fluorine-containing copolymers dissolved in organic solvents, said copolymers characterized by that
(1) they are soluble in organic solvents at low temperature,
(2) they require no specific hardener at the time of curing,
(3) they are cured at ordinary temperature,
(4) they are excellent in weathering resistance, water resistance, chemical resistance, solvent resistance and low friction characteristics after curing, and
(5) they are fluorine-containing copolymers which can be polymerized directly from monomer components,
[A] said copolymers being obtained by copolymerizing substantially
(a) fluoroolefin, PA1 (b) vinyl ether, and PA1 (c) an organosilicon compound having an olefinically unsaturated bond and a hydrolyzable group, PA1 number of said (a) to (c) in the copolymers, 30-70 mol % PA1 of (a), 20-60 mol % of (b) and 1-25 mol % of (c) and PA1 (Mn) of 3000-200000 as measured by gel permeation chromatography. PA1 (i) a fluoroolefin, PA1 (ii) an alkyl vinyl ether, PA1 (iii) vinyl ester of a carboxylic acid, and PA1 (iv) an organosilicon compound having an olefinically unsaturated bond and a hydrolyzable group;
[B] said copolymers comprising, based on the total mole
[C] said copolymers having a number average molecular weight
Though the above fluorine-containing copolymers have excellent characteristics, there is such a problem that when chlorinated fluoroolefins are used as (a) fluoroolefins, the resulting fluorine-containing copolymers are colored yellow, etc. Accordingly, coating compositions comprising the fluorine-containing copolymers dissolved in organic solvents have such a problem that when substrates or base materials are coated with the coating compositions, the resulting coating films are colored yellow.
Accordingly, it is an object of this invention to develop a process for producing a fluorine-containing copolymer which does not cause coloration and is soluble in organic solvents and curable at ordinary temperature.
Further, it has been found that coating compositions containing the above-described fluorine-containing copolymers are not considered to be fully satisfying with respect to the adhesion of their coating films to substrates or base materials. Namely, it has been found that such coating films are poor in adhesion strength as compared to coating films formed from epoxy resin primers after the lapse of a long period of time from application, although such coating films are excellent in adhesion to coating films formed from the epoxy resin primers for a short period of time from application.
Epoxy resins are most widely used as a vehicle for coatings. Hence, epoxy resin type coatings are often used together with other coatings, for example, by means of wet-on-wet coating. Accordingly, good adhesion to epoxy resins is often required in the field of coatings.
In practical coating work, there is often a possibility that a long time elapses before a top coating is applied after application of epoxy type coatings.
Accordingly, it is an object of the present invention to develop a fluorine-containing copolymer composition which gives a coating film excellent in adhesion to substrates.